High-intensity discharge (HID) lamps such as mercury vapor, metal halide, high-pressure sodium, low-pressure sodium lamps types are generally time consuming to ignite re-ignite. Typically, an ignition period of about twenty minutes may be needed in order for the lamp to sufficiently cool prior to attempting re-ignition.
Re-ignition may occur frequently, especially when the lamps are used with an unreliable power source. Generally, HID lamps will extinguish when power to the lamp is interrupted. Power interruptions of even a very short duration, e.g., tens of milliseconds, will often extinguish the lamp.
Since HID lamps are not illuminated during the lengthy ignition periods, they are often used in lamp system with an auxiliary lamp. The auxiliary lamp is responsive to the unlit HID lamp and accordingly provides light during the ignition period or whenever the HID lamp otherwise unavailable or unlit
These lamp systems generally include an alternating current (AC) power source which may have a variable amplitude. The auxiliary lamp, such as an incandescent lamp, generally requires a constant root mean squared (rms) voltage in order to operate properly. Accordingly, the lamp systems must include a control circuit for providing a constant root mean squared (rms) voltage.